In an uplink wireless system of a next generation mobile communication, a high transmission power efficiency needs to be attained in order to enlarge a communication area. Thus, a single-carrier (SC) system in which a peak to average power ratio (PAPR) is low is considered to be dominant. Also, in the next generation mobile communication, it is important to attain a high-speed data transmission. When an SC signal is used to carry out the high-speed data transmission, a problem of an inter-symbol interference (multipath interference) is caused. As a method of easily suppressing this multipath interference, a linear equalization is known. A frequency domain equalization is considered in which an equalizing process is executed in a signal process of a frequency domain so that an operational processing amount can be largely reduced. Also, in order to improve reception quality, a method is effective in which the SC signal is received by a plurality of receiving antennas, the respective reception signals are equalized, and an antenna diversity combining is carried out.
The frequency domain equalization typically uses pilot signals to estimate channel gains in a frequency domain and to calculate equalization weights. FIG. 1 shows one example of a format of a wireless frame signal when the frequency domain equalization is used. The wireless frame signal contains a plurality of blocks of a pilot signal and a data signal. In the frame shown in FIG. 1, a pilot signal block is located at the head and then a plurality of data signal blocks follow, A cyclic prefix (CP) is added to the head of each block, in order to avoid interference from a previous block at a time of a DFT (Discrete Fourier Transform) process. The CP is generated by copying a final portion data of each block to the foremost portion.
FIG. 2 shows a configuration of a conventional receiving apparatus. In the conventional receiving apparatus, the SC signal is received by N (N is an integer of 2 or more) receiving antennas, and then the multipath equalization and the antenna diversity combining are carried out in a frequency domain signal process. The conventional receiving apparatus contains receiving antennas 101-1 to 101-N, CP removing sections 102-1 to 102-N, DFT sections 103-1 to 103-N, reception filters 104-1 to 104-N, a channel estimating section 105, a noise power estimating section 106, a weight calculating section 107, an equalizing section 108 and an IDFT (Inverse Discrete Fourier Transform) section 109.
The receiving antennas 101-1 to 101-N are connected to the CP removing sections 102-1 to 102-N, respectively. The CP removing sections 102-1 to 102-N are connected to the DFT sections 103-1 to 103-N, respectively. The DFT sections 103-1 to 103-N are connected to the reception filters 104-1 to 104-N, respectively. The reception filters 104-1 to 104-M are connected to the channel estimating section 105, the noise power estimating section 106 and the equalizing section 108. The channel estimating section 105 is connected to the noise power estimating section 106 and the weight calculating section 107. The noise power estimating section 106 is connected to the weight calculating section 107. The weight calculating section 107 is connected to the equalizing section 108. The equalizing section 108 is connected to the IDFT section 109.
Each of the receiving antennas 101-1 to 101-N receives the SC signal. The CP removing sections 102-1 to 102-N receive the reception signals of the respective antennas and remove signal portions corresponding to the CP portions. The DFT sections 103-1 to 103-N receive the reception signals from which the CP portions have been removed, from the CP removing sections 102-1 to 102-N, and carry out the DFT processes of NDFT points (NDFT is an integer of 2 or more), and output, frequency domain signals transformed from the reception signals. The reception filters 104-1 to 104-N carry out bandwidth limitation of the frequency domain signals and carry out user separation and noise suppression. Typically, raised cosine roll-off filters (including a roil-off rate 0) are used in the reception filters 104-1 to 104-N. In the configuration shown in FIG. 2, the filtering of the frequency domain signal is carried out in the frequency domain signal process. However, the signal process of a time domain may be carried out prior to a DFT process.
The channel estimating section 105 carries out a correlating process between pilot reception signals and pilot reference signals in the frequency domain and consequently estimates channel gains of a desired user signal. FIG. 3 is a block diagram showing a configuration of the channel estimating section 105. The channel estimating section 105 contains a DFT section 111, a transmission/reception filter 112, a pilot reference signal generating section 113, a correlation calculating section 114 and a noise suppressing section 115.
The DFT section 111 is connected to the transmission/reception filter 112. The transmission/reception filter 112 is connected to the pilot reference signal generating section 113. The pilot reference signal generating section 113 is connected to the correlation calculating section 114. The correlation calculating section 114 is connected to the noise suppressing section 115.
The DFT section 111 performs a DFT process on a pilot code of a desired user signal to transform into the frequency domain signal. The transmission/reception filter 112 is applied to a portion of the frequency domain signal corresponding to the pilot code. The transmission/reception filter 112 is not required in case of the roll-off rate 0. The pilot reference signal generating section 113 uses the output of the transmission/reception filter 112 to generate a pilot reference signal that is used in correlation calculation with a pilot reception signal. In the pilot reference signal generating section 113, there are used a zero forcing (ZF) method of perfectly cancelling the code characteristics of the pilot reception signal, a minimum mean squared error (MMSE) method of suppressing noise increase in the correlation calculation, or a clipping method, A pilot reference signal X(k) (1·k·NDFT) in a subcarrier k when the ZF is used is represented by the following equation.
                              X          ⁡                      (            k            )                          =                                            C              *                        ⁡                          (              k              )                                                                                        C                ⁡                                  (                  k                  )                                                                    2                                              (        1        )            where C(k) indicates a pilot code characteristics of the output, of the transmission/reception filter 112, and a superscript * indicates a complex conjugate. The processes of the DFT section 111, the transmission/reception filter 112 and the pilot reference signal generating section 113 are sufficient to be carried out only once prior to the communication with a desired user. Also, a method of calculating a plurality of pilot reference signals in advance and storing in a memory and then selecting the pilot reference signal on the basis of a user is considered. The correlation calculating section 114 estimates channel gains in accordance with correlation calculation between the pilot reception signals and the pilot reference frequency domain signals. A channel estimation value vector H(k) (1·k·NDFT) to the subcarrier k is calculated by the following equation:H(k)=X(k)P(k)  (2)where P(k) indicates a pilot reception signal vector of the outputs of the reception filters 104-1 to 104-N. The noise suppressing section 115 suppresses noise in a channel estimation value of the output of the correlation calculating section 114 and improves the precision of the channel estimation value. As a specific operating method of the noise suppressing section 115, there are a method of calculating movement average of adjacent subcarriers, a method of transforming the channel estimation value into a time domain through an IDFT process once, and returning to the frequency domain by a DFT process after the removal of noise paths.
The noise power estimating section 106 estimates noise power from the channel estimation values of the pilot reception signal and the desired user signal in the frequency domain. FIG. 4 is a block diagram showing a configuration of the noise power estimating section 106. The noise power estimating section 106 contains a DFT section 121, a transmission/reception filter 122, a pilot signal replica generating section 123, a subtracting section 124, a noise power calculating section 125 and a subcarrier averaging section 126.
The DFT section 121 is connected to the transmission/reception filter 122. The transmission/reception filter 122 is connected to the pilot signal replica generating section 123. The pilot signal replica generating section 123 is connected to the subtracting section 124. The subtracting section 124 is connected to the noise power calculating section 125. The noise power calculating section 125 is connected to the subcarrier averaging section 126.
The DFT section 121 performs a DFT process on the pilot code of the desired user signal to transform into a frequency domain signal. The transmission/reception filter 122 is applied to a portion of the frequency domain signal of the pilot code. The transmission/reception filter 122 is not required in case of the roll-off rate 0. The processes of the DFT section 121 and the transmission/reception filter 122 may be carried out only once prior to the communication with the desired user. Also, a method is considered of calculating a plurality of filter output signals in advance, storing in the memory and selecting the filter output signal on the basis of the user. The pilot signal replica generating section 123 multiplies the output of the transmission/reception filter 122 and the channel estimation value to generate a pilot signal replica. The subtracting section 124 subtracts the pilot signal replica from the pilot reception signal in the frequency domain. The noise power calculating section 123 calculates power of the output of the subtracting section 124. The subcarrier averaging section 126 averages the noise powers over the subcarriers. Typically, the noise spectral is white Gaussian noise. Thus, a summation of the noise powers for all of subcarriers NDFT is averaged by the number NNB of subcarriers (NNB is an integer of 2 or more) corresponding to the noise bandwidth of the reception filter. A noise power σ2 is calculated by the following equation by using the pilot reception signal vector P(k), the channel estimation value vector H(k), the pilot code characteristics C(k), and the number NNB of subcarriers corresponding to the noise bandwidth of the reception filter:
                              σ          2                =                              1                          N              ×                              N                NB                                              ⁢                                    ∑                              k                =                1                                            N                DFT                                      ⁢                                                                                                p                    ⁡                                          (                      k                      )                                                        -                                                            H                      ⁡                                              (                        k                        )                                                              ⁢                                          C                      ⁡                                              (                        k                        )                                                                                                                        2                                                          (        3        )            
The weight calculating section 107 are supplied with the channel estimation value of the desired user signal and the noise power and calculates the equalization weights. Typically, MMSE is used in the weight calculating section 107. MMSE weights W(k) (1·k·NDFT) in a subcarrier m is calculated by the following equation by using the channel estimation value vector H(k) and the noise power σ2:W(k)=HH(k)[H(k)HH(k)+σ2I]−1  (4)where a superscript H indicates a Hermite conjugate, and I indicates a unit matrix. The equalizing section 108 receives the equalization weights calculated by the weight calculating section 107 and the reception signals bandwidth-limited by the reception filters 104-1 to 104-N, and they are multiplied for each subcarrier, and the multipath equalization of the reception signal and the antenna diversity combining are carried out in the frequency domain. When the data reception signal vector of the outputs of the reception filters 104-1 to 104-N is defined as D(k) (1·k·NDFT) and the weights calculated by the weight calculating section 107 is defined as W(k), an equalization signal Y(k) (1·k·NDFT) of the output of the equalizing section 108 is represented by the following equation:Y(k)=W(k)D(k)  (5)
The IDFT section 109 receives the equalization frequency domain signal that is the output of the equalizing section 108, and performs an IDFT process on NIDFT points (NIDFT is an integer of 2 or more) to transform into a signal of the time domain, and then outputs a demodulation signal.
The foregoing frequency domain equalization is also considered in the following document; “Frequency Domain Equalization for Single-Carrier Broadband Wireless Systems” (IEEE Commun. Mag., vol. 40, No. 4, pp.) by D. D. Falconer, S. L. Ariyavisitakul, A. Benyamin-Seeyar, and E. Eidson.
As mentioned above, in the conventional receiving apparatus, the SC signal is received by the plurality of receiving antennas, and the multipath equalization and the antenna diversity combining are carried out in the frequency domain. Consequently, in the isolated cell environment in which only the desired user signal exists, the superior performance can be achieved. However, in the multi-cell environment such as a mobile communication system, the users using the same frequency channel exist in the adjacent cell, and the signals of the users result in the interferences (other cell interferences). In the conventional receiving apparatus, those interferences are regarded as the noises, and the optimization is carried out in accordance with the MMSE. However, another cell interference is not always suppressed. Thus, when there is a severe other cell interference, the reception performance is degraded.